# Tag Info

39

Sound intensity is measured on the dB scale, which is a logarithmic scale of pressure. The "threshold of hearing" is given by the graph below: which tells you (approximately) that 0 dB is about "as low as you go" - the "threshold of hearing". Note that sound signal drops off with distance - we will have to take that into account in what follows. If you ...

24

In start-up and hover each blade produces more or less constant sound. But the sound is attenuated by distance and may not be the same in all directions. Therefore you hear it differently depending on the blade's position relative to you. So as the blades rotate, the sound you hear pulsates because the blades alternately get to positions where you hear them ...

8

It depends on your definition of "a sound". If a sound is not a sound unless it is perceived as a sound (that is, processed in the auditory system of a sentient being), then the answer is "no". If a sound is a coherent disturbance in the pressure distribution of the air, and this disturbance propagates through the medium "at the speed of sound", then the ...

4

First, assume a spherical helicopter... A helicopter isn't a sphere, or even close to one. Consequently, you get complicated acoustic effects where the pressure waves from the blades reflect off other parts of the helicopter, such as the tail boom. You also get intermittent reflection of the tail rotor sound off the main rotor blades, intermittent ...

4

We have to be careful about what we mean by "speed of light". It can mean two things: the speed at which light travels, which I'll write as $s_{light}$, and the maximum speed at which anything can possibly travel, which is written $c$. In our universe, in a vacuum, $s_{light}=c$, as far as we know. Now, no information can ever be transmitted faster than ...

4

Acoustic waves travel through a medium (air, water, metal, etc), there is no known medium through which light travels Both the speed of sound and the speed of light have fixed values regardless of the speed of their source Acoustic waves can be longitudinal (in gases) or transversal (in solids) whereas light is only transversal. You can measure acoustic ...

3

What you are probably observing is a stick-slip phenomenon (sometimes called a relaxation oscillator - think chalk on blackboard). The whole point is that high modulus ("not flexible enough" == high modulus) is just what you need to get such high frequencies: a small displacement must give rise to a large force. I must say I'm surprised you can do this by ...

3

Let's make the following assumptions: One needs to add $1$ log / $30$ min. to keep the fire burning, One log weight approximately $1$ kg. Under these conditions, with your 50% efficiency estimation, the power consumed by the fire is: $15$ x $10^6$ joules x $0.5$ / $1800$ seconds $=~ 4.2$ x $10^3$ W (which is consistent with the estimated heat output ...

3

Kudos to the question-asker for thinking about everything they read! :-) I was pleased to note that the author of the previous answer mentioned "audible" means "audible" to the human ear. Note also that "audible" also depends on the frequency a bit...generally-speaking, as humans, for high-frequency sounds we need them a little more intense if we're going ...

3

I suggest a totally different approach. But it's only a partial approach with much guesswork, too. The ear is able to perceive 20 µPa. (at 2 kHz). Of course you could calculate some pressure changes at the closing void, but these actually have nothing to do with the sound pressure at your ear drum. Let's do some energy calculations. 20 µPa at 1 cm² area at ...

2

It is generally caused by poor piloting technique. Blade-slap occurs when the helicopter is allowed to slowly drift downward while the pilot is still applying significant power. When blade-slap occurs, the pilot should either stop the descent, or lower the collective to enter a more positive descent.

2

I suppose you could say this is cheating, but you could surround the object emitting the sound with a perfect vacuum. Sound waves are vibrations in a medium; because a perfect vacuum has nothing in it, it cannot "conduct" (for lack of a better word) sound waves. You could attempt to levitate the object with magnets; because of Earnshaw's theorem, the setup ...

2

To first order, the speed of sound is not affected by pressure. Pressure waves can be shown to fulfill the D'Alembert wave equation $(c_S^2\,\nabla^2 - \partial_t^2)\psi=0$ where the wavespeed $c_S$ is given by: $$c_S = \sqrt{\frac{K}{\rho}}$$ where $K$ is the bulk modulus of the medium in question and $\rho$ its density. Now, for an ideal gas, the bulk ...

2

Sound is a pressure wave, and the generation of a pressure gradient requires atoms/molecules to move to create a density difference. No particle can move faster than light, so it's impossible to create a pressure gradient that propagates faster than light. Nathaniel's argument that sound waves could travel faster than light in a system like a Bose Einstein ...

2

Actually you do hear a constant whoosh (although often drowned out by engine noise) when you are directly below the helicopter. The tips of the blades cause a wave to propagate outward at the speed of sound. This wave does not have the same strength in all directions. If you are a distance away laterally, then you hear these waves in succession produced ...

2

For general audio programming or playback, 96kHz or 192hHz is simply useless. Indeed, the Nyquist theorem tells you that a signal can be exactly reproduced given that the sample rate is greater than the highest frequency contained in the original signal. The "excuse" of the slope of analog filter required after digital to analog conversion is no longer ...

1

For practical purposes, we can assume that the disappearance of a 1.2 cubic cm (1.2 ml) object gives a waveform that's very similar to the sudden appearance of a 1.2 ml object, except for the sign of the resulting pressure wave. Now we do have a simple means to create that effect: setting off gunpowder will suddenly produce a lot of gas. You'd need just a ...

1

The usual way that this phenomenon is described is when you hold a sea shell up to your ear, you can hear the sea What actually happens, according to this link, is that the vessel acts as a resonator / reflector for ambient noise - sounds that already present in the environment are amplified and stand out more. The "you hear your blood" myth is just ...

1

because the higher the sampling rate is the sloppier the (annalogue) filtering preceding the sampler can be to reduce the aliased noise/interference

1

The movement of air around the blades of the rotor produces a "white noise" due to turbulence. However, the "white noise" is actually "colored" and a certain band of frequencies predominates. As the blade approaches you there is a Doppler shift toward a higher frequency, and as it turns and goes away from you the Doppler shift reverses. It is this ...

1

The reason behind the hissing sound is that the temperature of the water droplet is much lower than the hot surface. As soon as the water droplet's base touches the hot surface it quickly evaporates but still the top part of the droplet is in liquid state and there is an opposition to th water-vapour coming from below. As the water-vapour couldn't vertically ...

1

There is a "universe" where the speed of sound is greater than the speed of light (or at least the speed of electromagnetic wave propagation), and that is inside conductors. In a conductor, the EM wave velocity is $$v = c \left(\frac{2 \omega \epsilon_0}{\sigma}\right)^{1/2},$$ where $\sigma$ is the conductivity and $\omega$ is the angular wave frequency. ...

1

Short answer: in phase radiates more in the far-field. But read the long answer. Long answer: this actually depends on the frequency and the distance between the sources. For a wavelength significantly larger than the distance between the sources, the pressures emitted by each source simply add up when in-phase; and they cancel when out-of-phase. But what ...

1

The following is not my own research, but taken from Randall Munroe' wonderful what-if "Glass Half Empty" where he describes a glass of water, bottom half filled with water, top half filled with vacuum (or: nothing). (edited to exclude other two glasses) But what if the empty half of the glass were actually empty—a vacuum? (Even a vacuum arguably isn’t ...

1

Acoustic Wave is a wave in which motion of one atom causes motion of another atom because it is lying next to it. Light is change in electric or magnetic field which further causes changing fields.

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